Radiation-curable liquid coating compositions are well known, and many of these involve diacrylate-terminated polyurethane oligomers, However, these known compositions are relatively hard when cured. One can, of course, dilute the diacrylate-terminated polyurethane with a large proportion of radiation-curable monomer of low T.sub.g (glass transition temperature) so that radiation exposure will cure the coating and produce a soft layer instead of a hard one. However, these soft layers have little strength and, hence, little utility (other than as pressure-sensitive adhesives).
In recent years, glass fiber has become important for the transmission of beams of light used for communication. These must be coated to protect the fiber surface against abrasion, to preserve strength and to prevent microbending when mechanically manipulated, and to prevent microbending when the coated fiber is subjected to changes in temperature. These complex characteristics demand an unusual coating, namely, one which is both soft and tough. Moreover, this toughness must be achieved without introducing stiffness which would cause microbending when the coating contracts at low temperature.